1. Field of the Invention
The invention relates to a method for the production of defoamer formulations using hydrophilic silica.
2. Description of the Related Art
In numerous liquid systems, more particularly aqueous systems which include surface-active compounds as desired or else unwanted constituents, it is possible for problems to occur as a result of foaming if these systems are contacted more or less intensely with gaseous substances, such as during the introduction of gases into wastewaters, during intense stirring of liquids, during distillation, washing or coloring operations or during dispensing operations, for example.
This foam can be controlled by mechanical means or through the addition of defoamers. Siloxane-based defoamer formulations have proven particularly appropriate here.
Defoamer formulations based on siloxanes are produced, for example, in accordance with U.S. Pat. No. 3,383,327 A by heating hydrophilic silica in polydimethylsiloxanes.
The production of defoamer formulations using polysiloxanes containing alkoxy or hydroxyl groups is likewise known. It involves, for example, using polysiloxanes which on average carry more than one alkoxy group per silicon atom (DE 2903725). This high concentration of functional groups, however, results in poor stability in the foaming media, leading in part to an action which rapidly subsides. This is desirable in those instances, for example, where the defoamer formulation is intended to act only during dispensing operations, but is not to suppress the foam later on (U.S. Pat. No. 4,101,443 A). For the majority of applications, however, it tends to be a disadvantage.
EP 163541 B1 describes a method for the production of defoamer formulations that react polysiloxanes having terminal hydroxyl groups with other siloxanes, under the action of catalysts, to form branched siloxanes. These branched siloxanes are then heated together with low molecular weight polysiloxanes (e.g., having a viscosity of 10-50 mm2/s) having two terminal hydroxyl groups and with hydrophilic silica. The resultant defoamer formulations have a very high viscosity, which is a disadvantage for practical use.
Patterson, Robert E. (Colloids Surf., A; 74(1); 115 26; 1993) likewise describes the production of defoamer formulations using hydrophilic silica and polydimethylsiloxanes containing terminal hydroxyl groups. A molecular weight of 4000 to 18,000 g/mol for this polysiloxane is said to be optimum for defoaming action in the black liquor produced during papermaking. As the skilled worker can easily calculate, this finding means that polydimethylsiloxanes in which 0.8 4 mol % of the siloxane units carry a silanol group are optimum in terms of defoamer activity.
Given that the activity of the defoamer formulations thus produced is deserving of improvement, it is often proposed that pretreated hydrophobicized silicas be used in place of the hydrophilic silica. According to U.S. Pat. No. 3,113,930 A, joint heating of the silica with the polydimethylsiloxane is advantageous even in processes using pretreated hydrophobicized silica. In the process of GB 1549884 A1 the pretreated hydrophobic silica is added at the emulsifying stage; EP 23533 B1 selects the appropriate pretreated hydrophobic silica on the basis of the specified methanol wettability; U.S. Pat. No. 4,145,308 A improves the incorporation of the pretreated hydrophobic silica by adding oleic acid.
According to DE 19504645 C1, pretreated hydrophobic silica is added to formulations prepared by heating hydrophilic silica in polydimethylsiloxanes, for the purpose of achieving improved activity of the defoamer formulations thus produced.
In U.S. Pat. No. 6,656,975 B1, pretreated hydrophobic silicas are used so that the contact angle between the defoamer formulation, an encapsulant, and an organic carrier liquid is less than 130°, thereby facilitating encapsulation and production of defoamer dispersions.
The methods for the production of defoamer formulations that use pretreated hydrophobic silicas have the disadvantage that they are uneconomical by virtue of the high price of the pretreated hydrophobic silicas. The defoamer formulations produced in accordance with the prior art have the further disadvantage, moreover, that they do not always exhibit a sufficiently long-lasting activity in highly foaming, surfactant-rich systems, or that on account of the high viscosity they are difficult to handle and in storage are not stable.